Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One commonly used duplexing method is Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In a FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using the H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be in the order of milliseconds. For a given communication system the frame duration may be fixed. For example, the frame duration may be 10 milliseconds.
In a TDD wireless communication system, a frame may be divided into a DL subframe and a UL subframe. In TDD wireless communication systems, the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel.
Orthogonal Frequency Division Multiplexing (OFDM) systems typically use Cyclic Prefix (CP) to combat inter-symbol interference and to maintain the subcarriers orthogonal to each other under multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail part of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred herein as an OFDM signal.
In addition to the purposes mentioned above, the CP is often used for frequency offset estimation at the receiver. Any frequency offset at the receiver relative to the center frequency of the transmitted signal causes the phase of the received signal to change linearly as a function of time. The two parts of an OFDM signal that are identical at the transmitter, i.e., the CP and the tail portion of the OFDM symbol, may undergo different phase change at the receiver due to the frequency offset. Therefore, the frequency offset can be estimated by performing correlation between the CP and the tail portion of an OFDM symbol. The angle of the CP correlation indicates the amount of phase rotation that is accumulated over the duration of an OFDM symbol. This accumulated phase rotation may then used for frequency offset estimation.
The frequency offset at the receiver during initial synchronization may be very high. Furthermore, since the client terminal may not be synchronized to any base station during initial synchronization, the OFDM symbol boundaries are not known to the client terminal. In wireless communication system deployments where frequency reuse is employed, the signals from several base stations may be superimposed. In some cases, the various base stations may not be time synchronized, i.e., the OFDM symbol boundaries for the different cells may not be time aligned. Even if the OFDM symbol boundaries are time aligned at the base stations, the propagation delays from different base stations to the client terminal may be different and therefore the OFDM symbol timing may not be time aligned at the client terminal receiver. Furthermore, in some wireless communication systems, such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) or LTE-Advanced wireless communication systems, an option of using different CP lengths exists and the exact CP in use may not be known a priori to the client terminal. In addition, the different base stations whose signals may be superimposed may be using different CP length. The overall received signal scenario is illustrated in FIG. 5. In case of TDD systems since the same frequency is used for transmit and receive, at power up the client terminal may not be aware of the boundary between DL and UL. In case of TDD systems, the significant power difference between DL and UL may create challenges in performing frequency offset estimation and may lead to inaccurate frequency offset.
Most wireless communication systems may employ some form of framing in the air interface. For example, 10 ms radio frames are used in the 3GPP LTE wireless communication systems and each radio frame comprises 10 subframes as shown in FIG. 6. Each subframe in turn consists of two slots and each slot consists of 6 or 7 OFDM symbols depending on the type of CP used as shown in FIG. 6. In the 3GPP LTE wireless communication system, two different CP lengths are used and they are referred to as Normal CP and Extended CP. In wireless communication systems, normally the specific air interface frame structure repeats itself over certain periodicity.
The 3GPP LTE wireless communication system uses the following synchronization signals to assist the client terminal in achieving time and frequency synchronization as well as the detection of physical layer cell identity:                Primary Synchronization Signal (PSS)        Secondary Synchronization Signal (SSS)The positions of the PSS and SSS are illustrated in the FIG. 7 for FDD air-interface of a 3GPP LTE wireless communication system. Note that the figure shows the position of the PSS and SSS for both the Normal CP and Extended CP. FIG. 8 illustrates the PSS and SSS positions for TDD air-interface of 3GPP LTE wireless communication system. The PSS and SSS for different cells may be different as described below.        
The different PSS and SSS are identified by different signal sequences used for transmission. Specifically, 504 physical cell identities are defined in 3GPP LTE wireless communication system specifications and they are organized into 168 groups with three identities in each group. The SSS sequence identifies the physical cell identity group and PSS identifies the physical cell identity within a group. Detecting a physical cell identity requires the detection of both the PSS and the SSS.
The PSS sequence in frequency domain is a length 63 Zadoff-Chu sequence extended with five zeros on each side and mapped to central 72 sub-carriers as shown in FIG. 9. The Direct Current (DC) subcarrier is not used. In 3GPP LTE wireless communication system three different PSS sequences are used with Zadoff-Chu root indices 24, 29 and 34 corresponding to cell identity 0, 1 and 2 respectively within the physical cell identity group. The exact PSS sequences are defined in the 3GPP LTE specification TS 36.211 “Physical Channels and Modulation.” The time domain PSS signal may be obtained by performing Inverse Discrete Fourier Transform (IDFT) of the frequency domain PSS. The two time domain PSS instances present within a 10 ms radio frame as shown in FIG. 7 and FIG. 8 are identical.
The SSS sequences in frequency domain are generated by frequency interlacing of two length-31 M-sequences X and Y, each of which may take 31 different M-values. The SSS is extended with five zeros on each side and mapped to central 72 sub-carriers as shown in FIG. 10. The DC subcarrier is not used. In 3GPP LTE wireless communication system, 168 valid combinations of X and Y are defined corresponding to 168 different physical cell identity groups. The time domain SSS signal may be obtained by performing IDFT of the frequency domain SSS. The two SSS sequences present in a 10 ms radio frame are different, namely SSS1 and SSS2 as shown in FIG. 7 and FIG. 8, which allows the client terminal to detect 10 ms radio frame timing from reception of a single SSS. The only difference between SSS1 and SSS2 is that the two M-sequences X and Y used in SSS1 are swapped in SSS2. Relative timing between SSS and PSS depends upon CP type and duplexing type as shown in FIG. 7 and FIG. 8.
Since OFDM symbol synchronization may be achieved at the end of PSS search, frequency domain processing may be employed for further analysis, such as SSS detection.
The SSS search has to handle timing and frequency offset ambiguities in addition to other system unknowns such as CP type and duplexing type. The relative timing (in terms of number of samples) between SSS and PSS varies depending upon CP and duplexing type. Multiple SSS search attempts may be required to resolve unknown system parameters such as CP type and duplexing type. If CP type is known prior to SSS detection, for example using CP correlator, corresponding SSS detection attempt may be skipped. The PSS detection may result in multiple possible PSS positions being detected due to the presence of multiple cells surrounding the client terminal.
Frequency offset in OFDM systems generally manifests itself in two components commonly referred as integer frequency offset and fractional frequency offset. Integer frequency offset refers to the frequency offset in terms of integral number of the subcarriers and the fractional frequency offset refers to the frequency offset remaining after excluding the integer frequency offset. In 3GPP LTE wireless communication system the frequency spacing between subcarriers is 15 kHz. Therefore, for example, a frequency offset of 35 kHz at the client terminal manifests itself as two subcarrier offset (30 kHz) and a fractional frequency offset of 5 kHz.
Fractional frequency offset may be compensated by estimating fractional frequency offset using conventional methods such as CP correlation. In conventional systems, the integer frequency offset may be detected in frequency domain by attempting to decode SSS with different assumptions about different SSS frequency bin positions.
The SSS detection requires selection of one out of the 168 possible valid combinations. When coupled with additional unknowns such as the integer frequency offset, the CP type, the duplexing type, the number of search candidates for SSS becomes excessive leading to high complexity and high power consumption. To handle integer frequency offset of ±30 kHz (two subcarrier spacing), the number of SSS frequency domain processing iterations required is five corresponding to the nominal position and two subcarrier offsets in both positive and negative directions. When the CP and duplexing type are not known, the number of combinations goes up to 20.